U.S. patent application number 12/306839 was filed with the patent office on 2009-08-06 for method and system for roasting a biomass feedstock.
Invention is credited to Raymond Guyomarc'h.
Application Number | 20090193679 12/306839 |
Document ID | / |
Family ID | 37688203 |
Filed Date | 2009-08-06 |
United States Patent
Application |
20090193679 |
Kind Code |
A1 |
Guyomarc'h; Raymond |
August 6, 2009 |
METHOD AND SYSTEM FOR ROASTING A BIOMASS FEEDSTOCK
Abstract
A method for roasting a load of plant biomass, includes the
following stages: generation of a treatment gas stream by a thermal
generating apparatus, the treatment gas stream being an inert gas
consisting essentially of CO.sub.2; generation of a bed of material
at high temperature, called the thermal base; treatment of the load
of biomass with the treatment gas stream, the treatment gas stream
being laden with gaseous components including a water steam and
volatile organic compounds originating from the load of biomass
during the treatment; and recycling of at least a portion of the
water steam by passing at least a portion of the laden gas stream
through the thermal base.
Inventors: |
Guyomarc'h; Raymond; (Saint
Theodorit, FR) |
Correspondence
Address: |
GREER, BURNS & CRAIN
300 S WACKER DR, 25TH FLOOR
CHICAGO
IL
60606
US
|
Family ID: |
37688203 |
Appl. No.: |
12/306839 |
Filed: |
June 28, 2007 |
PCT Filed: |
June 28, 2007 |
PCT NO: |
PCT/FR2007/001086 |
371 Date: |
December 29, 2008 |
Current U.S.
Class: |
34/467 ; 34/131;
34/132; 34/472; 34/477; 34/499; 34/514; 34/516; 34/77 |
Current CPC
Class: |
Y02E 20/12 20130101;
Y02E 50/10 20130101; C10L 9/083 20130101; Y02E 50/30 20130101; Y02E
50/15 20130101; C10L 5/44 20130101; F23G 7/105 20130101; C10B 53/02
20130101; Y02P 20/145 20151101; Y02E 50/14 20130101 |
Class at
Publication: |
34/467 ; 34/516;
34/472; 34/477; 34/514; 34/77; 34/131; 34/132; 34/499 |
International
Class: |
F26B 3/06 20060101
F26B003/06; F26B 11/04 20060101 F26B011/04; F26B 21/04 20060101
F26B021/04; F26B 21/14 20060101 F26B021/14; F26B 23/02 20060101
F26B023/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 2006 |
FR |
0605840 |
Claims
1-53. (canceled)
54. A method for roasting a load of plant biomass, comprising the
following stages: generation of a treatment gas stream by thermal
generating means, said treatment gas stream being an inert gas
consisting essentially of CO.sub.2; generation of a bed of material
at high temperature, called the thermal base; treatment of said
load of biomass with said treatment gas stream, said treatment gas
stream being laden with gaseous components comprising water steam
and volatile organic compounds originating from said load of
biomass during said treatment; and recycling of at least a portion
of said water steam by passing at least a portion of said laden gas
stream through said thermal base.
55. The method according to claim 54, characterized in that the
reactive thermal base is essentially composed of carbon-containing
constituents at high temperature obtained by combustion under
O.sub.2 of dried biomass.
56. The method according to claim 54, characterized in that the
thermal base is burning at a temperature that is controlled by
injection of oxygen in the center of said base.
57. The method according to claim 54, characterized in that it
additionally comprises combustion, during passage of the laden gas
stream through the thermal base, of organic gaseous components
originating from the load of biomass and present in said laden gas
stream, said combustion producing thermal energy that can be used
directly in the method and/or electric power by means of dedicated
systems.
58. The method according to claim 54, characterized in that it
further comprises recycling of the laden gas stream to recover the
gas that can be used in the treatment gas stream, said recycling
comprising a filtering of the laden gas stream, after the passage
of said stream through the thermal base.
59. The method according to claim 54, characterized in that the
generation of the gas stream for roasting comprises: a combustion
of roasted biomass under O.sub.2, said combustion producing a
combustion gas consisting essentially of CO.sub.2, and a
condensation of H.sub.2O components contained in the combustion
gas, in order to recover a residual gas consisting essentially of
carbon dioxide CO.sub.2.
60. The method according to claim 59, characterized in that the
residual gas travels through at least one heat exchanger to acquire
the treatment temperature, and then is returned to the treatment
cycle, to be used in the treatment of the load of biomass to be
roasted, the thermal energy necessary for raising the residual gas
to the treatment temperature is obtained by combustion of dried
biomass.
61. The method according to claim 54, characterized in that the
treatment gas stream is generated by combustion of a solid fuel,
said combustion also generating at least a portion of the thermal
base.
62. A system for roasting a load of plant biomass, comprising:
generating means provided for generating an inert treatment gas
stream consisting essentially of CO.sub.2 and a bed of material at
high temperature, called the thermal base; a treatment unit,
provided for receiving and subjecting said load of biomass to said
treatment gas stream, said treatment unit comprising a treatment
furnace and means for feeding the load of biomass into said
treatment furnace and for removing said load of biomass from said
treatment furnace; and gas exchange means provided for
communication between the generating means and the treatment
unit.
63. The system according to claim 62, characterized in that the
generating means comprise a thermal generator provided for
generating: at least a portion of the treatment gas stream, and at
least a portion of the thermal base said generator comprising a
thermal reactor or a solid-fuel furnace or alternatively a hybrid
device, allowing the combustion of a solid fuel and a gaseous
fuel.
64. The system according to claim 63, characterized in that the
thermal generator is equipped with a system for cooling by
circulation of a heat-transfer fluid.
65. The system according to claim 63, characterized in that the
thermal generator comprises a grate-type furnace provided for
receiving the thermal base and arranged for effecting the transfer
of the laden gases originating from the treatment unit.
66. The system according to claim 63, characterized in that the
thermal generator comprises means for injecting oxygen.
67. The system according to claim 63, characterized in that the
thermal generator comprises a chamber for post-combustion of
pyrolysis gases generated by the roasting of the load of biomass
and/or by the incomplete combustion of a solid fuel.
68. The system according to claim 63, characterized in that the
thermal generator comprises at least one heat exchanger, said heat
exchanger being provided for effecting heat exchange between either
a combustion gas and the treatment gas stream, or a fluid composed
essentially of saturated water steam and the treatment gas stream,
said fluid being essentially composed of water steam originating
either from the roasting of the load of biomass or from a cooling
circuit of a part of said system.
69. The system according to claim 62, characterized in that the
furnace is a cylindrical assembly comprising an inner cylinder
housed in an outer cylinder defining a space for treatment of the
load of biomass, said inner cylinder being provided with freedom to
rotate about a longitudinal axis relative to the outer cylinder and
receiving the load of plant biomass to be roasted.
70. The system according to claim 69, characterized in that the
inner cylinder comprises an inside wall which is perforated and at
least one protuberance on said inside wall, said protuberance
ensuring entrainment and mixing of the load of biomass during the
treatment.
71. The system according to claim 69, characterized in that the
outer cylinder has a heat-insulated shell and a solid inside wall
enveloping the inner cylinder and delimiting the treatment space of
the biomass feed.
72. The system according to claim 69, characterized in that the
treatment furnace comprises a deflector on almost the entire length
of the cylinder intended for directing the treatment gas stream
towards the lower portion of the treatment space so as to
distribute said stream onto all of the load of biomass.
73. The system according to claim 69, characterized in that the
treatment furnace has at least two brushes mounted in contact on
the one hand with the inside wall of the outer cylinder and on the
other hand between the outside wall of the inner cylinder so as to
delimit a zone for feed of the treatment gas stream into the
treatment furnace and a zone for extracting the gas stream after
treatment of the load of biomass, said brushes being arranged for
brushing the outside wall of the inner cylinder so as to dislodge
particles of the load of biomass clinging to the inner
cylinder.
74. The system according to claim 69, characterized in that the
treatment unit further comprises driving means arranged for
providing rotation of the inner cylinder about a longitudinal
axis.
75. The system according to claim 69, characterized in that one end
of the inner cylinder and of the outer cylinder is provided with an
opening permitting introduction of the load of biomass into the
inner cylinder before the treatment and extraction of said load of
biomass after the treatment, the other end being closed, said
opening being tightly closed by sealing means actuated by piston
means.
76. The system according to claim 69, characterized in that: in a
position called the charging position, the cylindrical assembly is
positioned vertically, the end having an opening in the inner
cylinder and outer cylinder that is at the top, in such a way that
the load of biomass to be treated can be fed into the inner
cylinder, in a position called the process position, the
cylindrical assembly is positioned horizontally, the opening of the
inner cylinder and outer cylinder is tightly closed by the sealing
means, and in a position called the discharging position, the
cylindrical assembly is positioned vertically, the end having an
opening of the inner cylinder and outer cylinder that is positioned
at the bottom, in such a way that the load of biomass treated is
collected in receiving means.
77. The system according to claim 69, characterized in that it
further comprises means for extracting the gas mixture from the
treatment space, provided so that said treatment space is kept at a
slight low pressure.
Description
[0001] The present invention relates to a method and a system for
roasting a load of plant biomass and more particularly a load of
wood.
[0002] The field of the invention is the field of roasting a load
of plant biomass and in particular a load of wood.
[0003] Plant biomass is a renewable raw material whose energy
potential, released on combustion, is very similar to that of coal.
Depending on its manner of thermal upgrading, plant biomass can
have an energy efficiency from 35% to 100%. This is due to the
"hydrophilicity" of the plant fibres, which soak up water, removal
of which consumes energy. The average lower heating value (LHV) of
dry plant biomass is about 18 100 kJ/kg.
[0004] By applying certain methodologies for thermal upgrading to
plant biomass, the final product can be brought up to the
theoretical value of its higher heating value (HHV), i.e. 32 750
kJ/kg. This increase in energy potential is peculiar to plant
biomass and more particularly to its chemical characteristics. This
increase in HHV per kilogram of final product is originating by
degradation of the starting biomass, at the expense of its original
intrinsic energy value. Thus, it is found that the HHV of the
combustible components of 1 kg of anhydrous biomass can reach an
average value of 23 600 kJ/kg. The same dried biomass, in the
current process conditions, has a usual average HHV of 19 100
kJ/kg.
[0005] One of the principles of optimization is to reduce the
amount of oxygen contained in the anhydrous matter, so as to
increase the percentage by weight of carbon. Roasting is one of the
methods currently used for achieving this result.
[0006] For roasting, a load of biomass must be heated to
temperatures between 280 and 320.degree. C. These are high
temperatures, and the energy consumed in heating a load of wood to
these temperatures is considerable and puts a strain on the overall
efficiency of the methods of roasting used at present.
[0007] One of the objectives of the invention is to propose a
method and a system for roasting a load of plant biomass offering
better yield than the methods and systems currently in use.
[0008] Another objective of the invention is to propose a method
and system for roasting a load of plant biomass which requires less
external energy supply for roasting a load of biomass than the
existing methods and systems.
[0009] Another objective of the invention is to propose a method
and a system for roasting that displays optimum environmental
performance, better than with the existing roasting systems.
[0010] The invention proposes overcoming the aforementioned
problems by a method for roasting a load of plant biomass,
comprising the following stages: [0011] generation of a treatment
gas stream by thermal generating means; [0012] generation of a bed
of material at high temperature, called the thermal base; [0013]
treatment of said load of biomass with said treatment gas stream,
said treatment gas stream being laden with gaseous components
comprising water steam and combustible pyrolysis gases originating
from said load of biomass during said treatment; and [0014]
recycling of at least a portion of said water steam by passing at
least a portion of said laden gas stream through said thermal
base.
[0015] The method according to the invention employs a thermal base
composed essentially of a bed of material at high temperature. This
layer of material at high temperature is then used for recycling
the treatment gas stream laden with gaseous components and in
particular water steam. The recycling of the treatment gas stream
makes it possible to recover some of the energy contained in the
laden gas stream by passing the laden gas stream through the
thermal base. Such a recycling can provide better roasting
efficiency/yield, a decrease in energy of the external energy
supply required for roasting, and less pollution in comparison with
the existing roasting methods and systems.
[0016] In a particular embodiment of the method according to the
invention, the thermal base is composed essentially of an optimized
load of plant biomass, combustion of which is carried out under
optimum conditions, allowing high temperatures to be obtained. This
layer of material at high temperature is then used for recycling
the gas stream used in the treatment method according to the
invention. This stream is laden with gaseous components after
treatment of the biomass that is to be roasted, in particular with
water steam, contained in the raw material, and organic compounds,
gasified in the course of roasting. By recycling the treatment gas
stream it is possible to recover some of the energy contained in
the water steam, extracted from the starting biomass. Passage of
the gas stream, laden with pyrolysis gases containing volatile
organic compounds (VOC), through the thermal base permits their
combustion at high temperature and utilization of the energy
released. This recycling optimizes the efficiency of roasting of
the plant biomass and protects the environment: [0017] the
recycling of the water steam extracted from the raw material and
recovery of the energy applied for its extraction, greatly reduces
the consumption of energy employed in the method, [0018] the
combustion of the organic compounds gasified in the course of the
roasting process can be complete. It is carried out while the
organic compounds are at high temperature, therefore in the gaseous
state, without any elementary condensation being possible. Their
combustion is stoichiometric and can be without impact on the
environment, [0019] the energy released by the combustion of the
organic compounds can satisfy the needs of the roasting process,
[0020] the residual energy is greater than that employed in
initiating the process and can benefit other applications, by
replacing the energy that they use or would use.
[0021] Advantageously the treatment gas stream is essentially
composed of CO.sub.2.
[0022] Moreover, the thermal base generated in the method according
to the invention is essentially composed of carbon-containing
constituents at high temperature.
[0023] The generation of the thermal base can comprise combustion
of roasted biomass under O.sub.2, said combustion producing
carbon-containing constituents at high temperature. The biomass
used as fuel can be of vegetable or animal type or of any other
type.
[0024] The reactive thermal base according to the invention can be
burning at a temperature that is controlled by injection of oxygen
in the centre of said thermal base. This injection of oxygen can
serve for controlling the temperature and the production of energy
within the thermal base.
[0025] The method according to the invention can comprise
co-generation of electricity from the water steam originating from
a cooling circuit or from any other circuit that can be employed in
the method according to the invention. The methods of co-generation
of electricity from water steam are well known to a person skilled
in the art.
[0026] The method according to the invention can in addition
comprise combustion, during passage of the laden gas stream through
the thermal base, of organic gaseous components originating from
the load of biomass and present in the laden gas stream, this
combustion producing thermal energy that can be used directly in
the method and/or electric power by means of dedicated systems. The
thermal energy produced can be used for roasting a new load of
wood.
[0027] Advantageously, the method according to the invention can
comprise recycling of the laden gas stream to recover gas that is
suitable for use in the treatment gas stream. The recovered gas can
be heat-transfer CO.sub.2.
[0028] This recycling can comprise filtering of the laden gas
stream, after passage of the stream through the thermal base. This
filtering can have the purpose of removing unburnt compounds during
passage of the laden gas stream through the thermal base.
[0029] In a particular version of the invention, generation of the
gas stream for roasting can comprise combustion of roasted biomass
under O.sub.2, this combustion producing a combustion gas
essentially comprising CO.sub.2. The roasted biomass can be plant
biomass. In a particular version of the method according to the
invention, the roasted biomass used for generating the gas stream
and/or for generating the thermal base can be roasted plant biomass
obtained by roasting plant biomass by the method according to the
invention.
[0030] After a combustion gas has been obtained, the method
according to the invention can comprise a preliminary phase of
condensation of elements contained in the combustion gas, for
recovery of a residual gas comprising carbon dioxide, this
condensation in particular having the purpose of removing the water
steam contained in the combustion gas.
[0031] The method according to the invention can in particular
comprise compression of the residual gas, for condensing and
recovering the carbon dioxide in liquid phase.
[0032] The residual gas can also travel through at least one heat
exchanger so that it is raised to the treatment temperature, and
can then returned to the treatment cycle, to be used in the
treatment of the load of biomass to be roasted.
[0033] The thermal energy required to heat the residual gas to the
treatment temperature can be obtained by combustion of roasted
biomass, in particular of roasted biomass obtained by the method
according to the invention, and by the combustion of the volatile
organic compounds.
[0034] In a particularly advantageous version of the invention, the
treatment gas stream can be generated by combustion of a solid
fuel, said combustion also generating at least a portion of the
thermal base.
[0035] According to another aspect of the invention, a system for
roasting a load of plant biomass is proposed, comprising: [0036]
generating means provided for generating a treatment gas stream and
a bed of material at high temperature, called the thermal base;
[0037] a treatment unit, provided for receiving said load of
biomass and subjecting it to said treatment gas stream, said
treatment unit comprising a treatment furnace and means for feeding
the load of biomass into said treatment furnace and for removing
said load of biomass from said treatment furnace; [0038] means for
gaseous exchange provided for communication between the generating
means and the treatment unit
[0039] The generating means comprise a device for combustion of a
solid fuel provided for generating the treatment gas stream by
combustion of said fuel.
[0040] The generating means also comprise a device for combustion
of a solid fuel, arranged in such a way that the combustion of said
solid fuel forms at least a portion of the thermal base.
[0041] In a particularly advantageous variant of the invention, the
generating means comprise a thermal generator provided for
generating at least a portion of the treatment gas stream, said
generator also being provided for generating at least a portion of
the thermal base.
[0042] The thermal generator can comprise a thermal reactor or a
solid-fuel furnace or a hybrid device, allowing the combustion of a
solid fuel, in particular of roasted plant biomass, this combustion
producing, on the one hand, a combustion gas stream of which at
least a part can be used as treatment gas stream, and on the other
hand, carbon-containing constituents at high temperature, at least
a portion of which can be used for producing the bed of material at
high temperature called the thermal base.
[0043] Advantageously, the thermal generator can be equipped with a
system for cooling by circulation of a heat-transfer fluid. The
generator can comprise double walls, between which the
heat-transfer liquid, for example water under pressure, can
circulate. The heat-transfer liquid can also be sprayed onto the
walls of the thermal generator.
[0044] In a particular variant of the invention, the thermal
generator can comprise a grate-type furnace intended to receive the
thermal base and arranged for effecting transfer of the laden gases
originating from the treatment unit.
[0045] The grate-type furnace can advantageously be equipped with a
system for cooling by circulation of a heat-transfer fluid in the
furnace grate.
[0046] The thermal generator can also comprise means for injecting
oxygen. The injection of oxygen can, on the one hand, provide
combustion of a solid fuel intended for the generation of the
treatment gas stream and/or of the thermal base, and on the other
hand for regulating the temperature in the thermal base.
[0047] The thermal generator can in particular comprise a chamber
for post-combustion of pyrolysis gases generated by the roasting of
the load of biomass and/or by the incomplete combustion of a solid
fuel. This post-combustion chamber is employed in particular for
combustion of the volatile organic compounds and the pyrolysis
gases.
[0048] Advantageously, the thermal generator can comprise at least
one heat exchanger, said heat exchanger being provided for
effecting heat exchange between either a combustion gas and the
treatment gas stream, or a fluid composed essentially of saturated
water steam and superheated water and the treatment gas stream,
this fluid being essentially composed of water steam that comes
either from roasting of the load of biomass, or from a circuit for
cooling a part of the system.
[0049] The treatment furnace according to the invention can be a
cylindrical assembly comprising an inner cylinder housed in an
outer cylinder defining a space for treatment of the load of
biomass, said inner cylinder receiving the load of plant biomass
that is to be roasted.
[0050] The inner cylinder can in particular be provided with
freedom to rotate about a longitudinal axis relative to the outer
cylinder.
[0051] The wall forming the inner cylinder can advantageously be
perforated, in such a way that, on the one hand, the treatment gas
can be fed into this cylinder and come in contact with the load of
biomass to be treated, and on the other hand, the laden gas can
leave this cylinder after treatment of the load of biomass.
[0052] Moreover, the inner cylinder can comprise at least one
protuberance on its inside wall, over almost the entire length of
the inside wall, said protuberance ensuring the entrainment and
mixing of the load of biomass during the treatment. Contact of the
treatment gas with the load of biomass is thus facilitated and
treatment of the load of biomass is improved. After treatment,
mixing of the treated load of wood facilitates the release of the
laden treatment gas.
[0053] In an advantageous version of the system according to the
invention, the outer cylinder can comprise a heat-insulated shell
limiting the heat losses and increasing system security.
[0054] The outer cylinder can moreover comprise a solid inside wall
enveloping the inner cylinder and delimiting the space for
treatment of the load of biomass. This inside wall defines the
treatment space that is in contact with the various gas
streams.
[0055] Advantageously, the treatment furnace can comprise a
deflector on almost the entire length of the cylinder, intended for
directing the treatment gas stream towards the lower portion of the
treatment space so as to distribute said stream onto all of the
biomass load.
[0056] The treatment furnace can comprise at least two brushes
mounted in contact, on the one hand, with the inside wall of the
outer cylinder, and on the other hand, between the outside wall of
the inner cylinder in order to delimit a zone for feed of the
treatment gas stream into the treatment furnace and a zone for
withdrawal of the gas stream after treatment of the load of
biomass.
[0057] These brushes can advantageously be arranged for brushing
the outside wall of the inner cylinder so as to dislodge particles
of the load of biomass retained on the inner cylinder.
[0058] The treatment furnace additionally comprises a pipe for feed
of the treatment gas stream into the treatment space. This pipe for
feed of the gas stream can be heat-insulated by the methods and
systems known by a person skilled in the art.
[0059] The treatment furnace also comprises a pipe for withdrawal
of the treatment gas stream. This pipe for withdrawal of the
treatment gas stream can be heat-insulated.
[0060] The treatment furnace can advantageously comprise a pipe for
injecting liquid CO.sub.2 into the treatment zone. This pipe for
injecting CO.sub.2 is provided for safety reasons and for
regulating the temperature within the space for treatment of the
load of plant biomass.
[0061] In a particular embodiment the treatment unit can comprise
driving means arranged for providing rotation of the inner cylinder
about a longitudinal axis. These rotating means, by effecting
rotation of the inner cylinder, provide mixing of the load of
biomass present in the inner cylinder.
[0062] According to a particular embodiment of the system according
to the invention, one end of the inner cylinder and of the outer
cylinder is provided with an opening for feeding the load of
biomass into the inner cylinder before treatment and for extracting
the load of biomass after the treatment, the other end being
closed.
[0063] During treatment of the load of biomass, this opening is
tightly closed by piston-actuated sealing means.
[0064] The treatment unit can moreover comprise means for
horizontal positioning of the treatment furnace. These positioning
means allow the treatment unit to move to a horizontal position,
said position being maintained during treatment of the load of
wood.
[0065] The treatment unit can moreover comprise means arranged for
rotation of the cylindrical assembly about a horizontal axis. These
rotating means are arranged for positioning the treatment unit in
particular positions for charging and discharging of the load of
biomass.
[0066] The treatment unit can advantageously comprise means for
receiving the load of biomass after treatment. These receiving
means can comprise a receiving tank or a receiving wagon.
[0067] In one position, called the charging position, the
cylindrical assembly is positioned vertically, the end having an
opening in the inner and outer cylinders being at the top, in such
a way that the load of biomass to be treated can be fed into the
inner cylinder. This position can be used advantageously for
dismounting the cylindrical assembly, or one of the cylinders of
the treatment unit, for maintenance operations. This position
permits very practical and very ergonomic charging of the wood load
directly into the inner cylinder.
[0068] In one position, called the discharging position, the
cylindrical assembly is positioned vertically, the end having an
opening in the inner and outer cylinders being placed near the
bottom, in such a way that the treated biomass load is collected in
receiving means. This discharging position permits practical and
simple discharging of the load of biomass into means for receiving
the load of biomass.
[0069] In another position, called the processing position, the
cylindrical assembly is positioned horizontally, the opening in the
inner and outer cylinders being tightly closed by the sealing
means.
[0070] The system according to the invention can moreover comprise
means for extracting the gas mixture from the treatment space for
keeping said treatment space permanently at low pressure. These
extracting means can comprise means allowing aspiration of the
treatment gas stream and can be positioned downstream of the
treatment space and connected to the pipe for withdrawing the laden
gas stream.
[0071] The system according to the invention can further comprise a
water steam generating device, utilizing the thermal energy from
any element of the system.
[0072] Advantageously, the system according to the invention can
comprise means for co-generation or for tri-generation of energy
from the recovered thermal energy.
[0073] The system according to the invention can in addition
comprise means for storage and/or distribution of O.sub.2 and means
for storage and/or liquefaction and/or distribution of
CO.sub.2.
[0074] Other advantages and characteristics of the invention will
become apparent on examination of the detailed description of an
embodiment which is in no way limitative, and the attached drawings
in which:
[0075] FIG. 1 is a diagrammatic representation of a cross-sectional
view of a treatment unit according to the invention;
[0076] FIG. 2 is a diagrammatic representation of a view in
longitudinal section of a treatment unit according to the
invention;
[0077] FIG. 3 is a diagrammatic representation of a treatment unit
in side view of a treatment unit according to the invention;
[0078] FIG. 4 is a diagrammatic representation of a treatment unit
of a view of the opposite side of a treatment unit according to the
invention;
[0079] FIG. 5 is a diagrammatic representation of a treatment unit
according to the invention viewed from the front;
[0080] FIG. 6 is a diagrammatic representation of a treatment unit
according to the invention viewed from the rear;
[0081] FIG. 7 is a diagrammatic representation of a top view of a
treatment unit according to the invention;
[0082] FIG. 8 presents several diagrammatic representations of the
treatment unit in swivelling mode, all said representations being
based on a side view of the treatment unit;
[0083] FIG. 9 is a diagrammatic representation of a system for
roasting according to the invention;
[0084] The example discussed below is a particular, non-limitative
example of the present invention. It relates to a system for
roasting a load of plant biomass and more particularly a load of
wood.
[0085] The system described in the present example comprises a
treatment unit 1 as shown in FIGS. 1 to 7 in various views. These
various diagrams show a treatment furnace 10 which is in the form
of a cylindrical assembly, comprising an outer cylinder 11 and an
inner cylinder 12. The treatment furnace 10 is able to swivel about
a horizontal axis A2 for charging the wet wood load B1 and for
discharging the roasted wood load B2. In addition, the inner
cylinder 12 is able to rotate, relative to the outer cylinder 11,
about the longitudinal axis A1 shown in FIG. 2. The outer cylinder
11 is fixed. The inner cylinder 12 is formed from a perforated wall
and a solid bottom, into which the wet wood load B1 to be treated
is fed. FIG. 1 shows the wood load B as it is entrained by the
rotation of the inner cylinder 12. The inner cylinder 12 has
protuberances 121 in the treatment zone, which ensures entrainment
and mixing of the wood load B to be roasted.
[0086] The outer cylinder 11 has a solid inner wall, which envelops
the perforated inner cylinder 12 for roasting, and it is in the
zone delimited by this cylinder 11 that the stream of heat-transfer
gas (composed essentially of CO.sub.2) is introduced and withdrawn.
This zone is called the treatment space.
[0087] The treatment space is separated into two parts, 13 and 14,
by special high-temperature brushes 18. This space is thus divided
into two zones, namely: [0088] the inlet zone 13 corresponding to
the zone for introduction of the heat-transfer gas stream which
will travel through the load of biomass B; [0089] the outlet zone
14 corresponding to the zone for withdrawal of the laden gas
stream, composed of the heat-transfer CO.sub.2 and moisture and/or
pyrolysis gases extracted from the wood to be roasted.
[0090] The inlet zone 13 of the treatment gas stream also
corresponds to a zone for expansion and distribution of the hot,
dry heat-transfer CO.sub.2, the gas being distributed over the
entire outside surface of the rotating perforated inner cylinder 12
corresponding to the surface occupied by the load of wood to be
roasted.
[0091] The outlet zone 14 corresponds to the treatment space not
occupied by the wood load to be roasted downstream of the
industrial brushes 18. The hot, dry heat-transfer CO.sub.2 that is
fed into zone 13 passes through the wood to be roasted, in which it
will transfer its thermal energy to the wood load B by the three
known methods of heat transfer: [0092] conduction [0093] convection
[0094] radiation
[0095] But also by a fourth heat transfer method: that of osmosis
of the CO.sub.2 with the moisture contained in the biomass that is
to be roasted. After passing through the wood, the heat-transfer
gas stream entrains: [0096] the moisture evaporated from the wood,
during the dehydration phase [0097] the pyrolysis gases "VOC"
during the roasting phase.
[0098] The laden gas stream is then drawn through the perforations
of the inner cylinder 12 and is extracted via the outlet pipe
16.
[0099] The brushes 18 are arranged over the entire length of the
cylinder of the inside wall of the outer cylinder 11 at the
junctions of the inlet zone 13 and outlet zone 14. These industrial
brushes 18 are detachable so that they can be replaced if they are
worn; their role is to separate the treatment space into two zones
and to provide constant brushing of the outside wall of the inner
cylinder 12 to dislodge particles of wood that could be retained by
the perforations present on said cylinder 12.
[0100] The treatment furnace 10 also comprises a heat-insulated
outer shell, which corresponds to the outside wall 111 of the outer
cylinder 11. The furnace 10 can also have a buffer zone 112, and
this too can be heat-insulated.
[0101] The treatment furnace 10 also comprises an inlet pipe 15 of
the high-temperature heat-transfer gas stream, this pipe 15 and the
opposite outlet pipe 16 for the laden gas stream are integral with
the outer cylinder 11. They swivel in the supports 191 when the
roaster is tilted for charging with wet wood B1 or for discharge of
roasted wood B2. The roasted wood B2 is received at the end of
treatment in the detachable tank 17.
[0102] The outlet pipe for the laden gas stream (i.e. the treatment
gas stream and, depending on the phase of the treatment, the
moisture from the wood or the pyrolysis gases) can be supplemented
with an electric extractor (not shown) which maintains a constant
low pressure in the roaster.
[0103] The pipes 15 and 16 are heat-insulated. They are connected
to fixed pipelines (not shown), for feed of the heat-transfer gas
stream and for extraction of the treatment gas, from the recycling
loop leaving the heat exchangers and returning to the thermal
generator.
[0104] The treatment furnace 10 also comprises at least one
deflector 132 which directs the heat-transfer gas stream to the
bottom portion of the inner cylinder 12 containing the wood load B
to provide distribution throughout the mass of wood to be
roasted.
[0105] The treatment furnace 10 further comprises a pipe 131 for
injection of liquid CO.sub.2, the purpose of said pipe being:
[0106] to ensure safety of the treatment unit 1 by neutralizing any
risk of ignition of the biomass during roasting, [0107] to provide
cooling of the roasted wood at the end of treatment, to lower its
temperature to values below any possibility of self-ignition in the
open air. During the cooling phase: [0108] the inner cylinder 12
continues rotating, for even distribution of the liquid CO.sub.2
that will capture, through the hot roasted wood, its latent heat of
evaporation; [0109] feed of heat-transfer CO.sub.2 is cut off;
[0110] extraction of the laden gas stream continues until the
desired temperature is obtained.
[0111] The pipe 131 for injection of liquid CO.sub.2 is connected
to a system for distribution of liquid CO.sub.2 under pressure,
shown diagrammatically in FIG. 9, and an automatic safety valve
(not shown) provides the safety and cooling functions in case of a
power cut.
[0112] The treatment unit 1 comprises fixed supports 19 for the
roasting furnace 10 which receive the means 191 and 192 that enable
the furnace 10 to swivel about the axis A2. The height of said
supports 19 permits tilting of the roasting furnace 10, above the
receiving tank 17 for the roasted wood during its rotation in the
vertical positions for charging and discharging of the roasted wood
load.
[0113] The swivelling of furnace 10 about the axis A2 is provided
by the rotating means 191 and 192 which can comprise a chain-driven
electric mechanism or any other known means, positioned on one of
the supports. The pipes 15 and 16 are the shafts for support and
swivelling/rotation of the roasting furnace.
[0114] The supports 19, as shown in FIG. 7, are in the form of a
chassis stabilized by at least three feet: [0115] two feet
supporting the rotating means 191 and 192, [0116] at least one foot
receiving the sealing means 23 and 24 of the open end as well as
the means 21 and 22 for horizontal positioning of the furnace
10.
[0117] The sealing means comprise a piston 24 which pushes a plug/a
door 23 against the open ends of cylinders 11 and 12 of the
roasting furnace 10 to close them tightly during treatment of a
wood load B.
[0118] The means for horizontal positioning of the roasting furnace
comprise: [0119] a plate 21 which is dimensioned to the diameter of
the roasting furnace and corresponds to the useful distance for
assuming the following positions: [0120] a first horizontal
position when a load of wood is undergoing treatment; [0121] a
second tilted position when it releases the roasting furnace,
either for discharge of a roasted wood load or for charging a load
of wood to be roasted. In the discharge position, plate 21 connects
to the receiving tank 17 and thus serves as a chute for receiving
the roasted wood load, once door 23 is opened. [0122] a piston 22
which controls the positions of plate 21.
[0123] The rotation of cylinder 12 within the treatment furnace 10
about axis A1 is provided by a mechanism with an electric motor
25.
[0124] FIG. 8 shows the treatment/roasting furnace 10 in the
swivelling positions which allows it to be positioned: [0125] in
the charging phase; [0126] in the roasting phase; and [0127] in the
discharge/extraction phase.
[0128] FIG. 8 shows the different positions of furnace 10 during a
complete treatment cycle: [0129] positions 80 and 81: The furnace
tilts to the vertical position, with the open end EO at the top,
and the closed end EF at the bottom, for feed of a new charge of
wood B to be roasted; [0130] position 82: furnace 10 tilts to its
position for roasting a load of wood, corresponding to position 84;
[0131] position 82 and 83: plate 21 returns to its horizontal
position and controls the positioning of furnace 10 in the roasting
position; [0132] position 83 and 84: door/plug 23 closes
hermetically, treatment of the load wood B can be carried out;
[0133] position 85: door/plug 23 opens, roasted wood can flow onto
the positioning plate 21; [0134] position 86: positioning plate 21
swivels to form a chute to the receiving tank 17. The roasting
furnace 10 can swivel; [0135] positions 87 and 88: roasting furnace
10 tilts to the vertical position, with the open end EO at the
bottom and the closed end EF at the top, for
transfer/extraction/discharging of roasted wood into the receiving
tank 17.
[0136] An example of a system for roasting a load of wood according
to the invention and its principle of operation are shown in FIG.
9. It comprises a roasting unit 1 as described above. The roasting
unit 1 receives the wood to be roasted B1, in the form of forestry
chips, by-products and related shredded products as well as ground
products with the same dimensions as sawdust.
[0137] The roasting system as shown in FIG. 9 also comprises a
thermal generator G. It is a thermal generator with boiler for the
production of high-pressure water steam and exchangers: gas/water
and gas/gas. The thermal generator G comprises: [0138] a thermal
reactor R of high efficiency. This reactor receives at least a
portion of the roasted wood B2 on a grate in order to form a bed of
solid fuel, which will be supplied with industrial oxygen as
supporter of combustion. This is the reactive "thermal base". This
"thermal base" is fed continuously with roasted wood, with
injection of O.sub.2 to produce a seat of combustion for a
high-temperature reactor. By controlling the injection of O.sub.2,
combustion of the "thermal base" is arranged to provide the
reactions required for: [0139] the thermal capacity for roasting
the wood, and optionally [0140] high-output water steam generation,
The cycle is organized for optimum production of thermal energy in
all the sources of the system, as well as the recycling and optimum
utilization of the energy generated by the method. [0141] a heat
exchanger E1, or water steam boiler: the water for thermal control
of the reactor walls is vaporized in this exchanger and then
injected into a water steam-turbine alternator and/or a storage
tank. The temperature and pressure of this water steam are
determined by the combustion temperature in the reactor R. All of
the parameters can be adjusted by modifying the thermal reaction of
the reactor by controlling the injection of O.sub.2. The
heat-transfer gas stream acquires its thermal load in an optimum
manner in this exchanger, for rapid exchange of sensible heat.
[0142] a gas/gas heat exchanger E2, for the laden gas stream,
combustion gas+heat-transfer gas laden with recycle CO.sub.2 which
then acquires its temperature of heat-transfer treatment gas stream
for roasting.
[0143] The system also comprises a gas/gas heat exchanger E3 (whose
purpose is to cool the combustible gases) in which the laden gas
stream exchanges the residual thermal capacity (that it acquired on
passing through the thermal reactor R and the residual heat from
the treatment furnace 10) with the cold, dry heat-transfer CO.sub.2
arriving from the dehydrator D. At least a portion of the water
steam (extracted from the load of wood to be roasted) is condensed
in this exchanger E3, its latent heat of condensation thus being
recovered.
[0144] In FIG. 9, F represents a dust filter. The laden gas stream
coming from the thermal reactor R is likely to be carrying carbon
dust, which will be trapped here, and this combustible dust is then
burnt with the biomass from reactor R.
[0145] Also in FIG. 9, GR and D represent a system for dehydration,
which is made up of two elements: [0146] the refrigeration unit GR
and [0147] the refrigerant condenser D where the gas mixture, laden
with water steam (extracted from the load of wood to be roasted)
from the roaster 1, is cooled and dried.
[0148] The roasting system advantageously comprises an O.sub.2
system for storage and distribution of the oxygen for supporting
combustion. The consumption of oxygen, as supporter of combustion
of the "thermal base", is related to the power used.
[0149] Finally the system can comprise a water steam generating
device VAP. The production of water steam has several possible
functions: [0150] 1. High-pressure water steam for a
turboalternator; [0151] 2. Water steam for energy storage; or
[0152] 3. Water steam for dissipating excess energy: in this device
the water recovered in the dehydrator D, during the dehydration
phase, is evaporated in exchanger E1, which is "open-mouthed" i.e.
open to the open air (or escaping freely). The water steam is
evacuated as it is produced. This system makes it possible to
absorb the excess energy during the roasting phase, in production
of CO.sub.2, it has the advantage of not being under pressure in
its evaporation circuit and the water steam generated is evacuated
to the ambient air. Any existing system for evacuation can be
employed, provided the excess energy is thus evacuated. This system
also has the advantage that it can be reversible and can be used in
one of the other two configurations (1 and 2 above).
[0153] The generator G and more particularly the reactor R,
comprises a grate-type furnace, which can be cooled conventionally
by circulation of water or by any hydraulic heat-transfer means.
The walls of the generator are also under thermal control, cooled
by the same method, or configured so as to optimize heat exchange
to the heat-transfer gas stream. The grate of the furnace receives
the fuel in a bed of solid fuel. This bed is preferably composed of
roasted plant biomass, densified or not, but can be pre-dried,
anhydrous plant biomass, or a compacted form of plant biomass.
Combustion is preferably effected with oxygen injected into the
furnace, at the reactive centre of the biomass.
[0154] The generator can also comprise a chamber for
post-combustion of the pyrolysis gases generated by the roasting
and combustion of the biomass on the grate of the furnace. The
system is then dedicated purely to the optimum thermal upgrading of
the roasting process.
[0155] Combustion of the bed of combustible biomass can take place
under O.sub.2 as the supporter of combustion or under air as the
supporter of combustion, said reactions are then carried out
"ALTERNATELY and SEPARATELY", to produce a bed of embers and thus
form the "thermal base", through which the gases extracted from the
roasting furnace 10 pass, and are purified there. The gas mixture,
combustion gas in below-stoichiometric conditions and pyrolysis
gases, is thus brought to the ad hoc temperature for stoichiometric
post-combustion.
[0156] The bed of solid fuel, called the thermal base, is composed
of anhydrous biomass, preferably roasted and therefore with higher
concentration of vegetable carbon. Combustion of the thermal base
under O.sub.2 as supporter of combustion permits fine control of
combustion. This bed of roasted biomass burns at high
temperature.
[0157] The first objective of the generator G is to produce, for
the roasting system: [0158] CO.sub.2, which constitutes the
heat-transfer gas stream used in the process (in this case the
supporter of combustion of the solid biomass fuel is industrial
oxygen) and [0159] the heat used for roasting.
[0160] Combustion of the roasted biomass under O.sub.2 is complete
and only produces CO.sub.2. The CO.sub.2 introduces a process of
heat transfer supplementary to the known processes of heat
transfer. This process of heat transfer is specific to the raw
material, composed of plant biomass, and is osmosis of the CO.sub.2
with the moisture contained in the biomass.
[0161] This osmosis is made possible by the phytobiological
symbiosis of the CO.sub.2 and the "biomass" material: [0162] C and
O.sub.2 are the essential constituents of the "biomass" material,
the CO.sub.2 (atmospheric) being its natural ingredient [0163] the
water contained in the material is the natural solvent of the
CO.sub.2
[0164] The second purpose of said generator G is to carry out
complete combustion of the combustible constituents generated by
the process, for upgrading its thermal potential, in order to:
[0165] optimize the energy efficiency of the process and [0166]
produce a process gas which is: [0167] recyclable by the process,
in the system or, [0168] non-polluting, in the case when an excess
of CO.sub.2 would, for economic reasons, be discarded to
atmosphere. It will be possible, in this case, for combustion to be
carried out under air as supporter of combustion.
[0169] The CO.sub.2 produced by combustion of plant biomass is
considered to be neutral since the biomass is renewable and the
same amount of atmospheric CO.sub.2 will be used for growth of the
same amount of biomass. Combustion of the biomass must therefore be
complete so that the discarded CO.sub.2 does not contribute to the
greenhouse gases.
[0170] The CO.sub.2 resulting from combustion of the biomass under
O.sub.2 passes through the primary pipes of the heat exchangers
where it will transmit its heat to the heat-transfer components of
the system: [0171] heat-transfer gas stream for treatment of the
wood to be roasted, [0172] Optionally, water to be superheated
and/or evaporated, for storage of thermal energy and/or
co-generation of electricity.
[0173] Once cooled to a temperature below the temperature of
condensation of the water steam contained in the gas<70.degree.
C., the dewatered CO.sub.2 is filtered (to trap the carbon
particles that could have been entrained in the stream). It is then
in the required conditions for utilization as heat-transfer means
for roasting the wood load B.
[0174] This gas is then transferred to the heat exchanger to be
brought to the temperature required for the roasting treatment. The
heat-transfer CO.sub.2 is then fed into the roasting furnace 10,
where it transmits its heat capacity to the wood B to be roasted.
Heat transfer to the wood, according to the four processes of heat
transmission defined above, raises its temperature and enables the
moisture contained in the wood B to be evaporated.
[0175] The laden gas mixture (heat-transfer CO.sub.2+water steam
and then heat-transfer CO.sub.2+VOC, extracted from the wood to be
roasted) is then withdrawn from the roasting furnace 10 and
transferred to the thermal generator to be: [0176] purified through
the burning "thermal base" [0177] dried in an exchanger/dehydrator
and filtered [0178] then transferred to the heat exchanger where it
acquires its new charge of process heat.
[0179] Everything takes place with continuous recycling, up to the
end of roasting.
[0180] The roasting cycle of this design is carried out in
conditions of total self-production of energy; only the purchase of
oxygen as supporter of combustion and of the electricity used by
the system (unless it is self-produced) have to be taken into
account in this part of the direct operating costs.
[0181] In another operational configuration, the heat-transfer gas
circuit is arranged as a closed loop, which contains the volume of
heat-transfer CO.sub.2 used for the heat exchanges in the process.
The heat-transfer gas stream travelling in this circuit no longer
passes through the "thermal base" reactor of the generator during
the dehydration phase, but only through the exchangers where:
[0182] the heat-transfer gas stream acquires its process heat
capacity, [0183] the laden gas mixture, heat-transfer
CO.sub.2+water steam extracted from the wood to be roasted, is
dried.
[0184] When the wood is dried, the laden gas mixture extracted from
the wood to be roasted, heat-transfer CO.sub.2+volatile organic
compounds (VOC), then travels through the thermal reactor of the
generator, for stoichiometric combustion of the VOC under O.sub.2
as supporter of combustion. The combustion gases are composed
essentially of CO.sub.2. If they are at the right temperature for
roasting, then they are fed directly into the roasting furnace 10,
without any other form of treatment. If their heat capacity is
excessive, then they are discharged in a heat exchanger to the
benefit of a heat-transfer component dedicated to another
application and/or for buffer thermal storage.
[0185] In this case combustion of the VOC can take place under
atmospheric (air) supporter of combustion, this solution only being
envisaged if the combustion gases are not used in the roasting
process: too much heat-transfer CO.sub.2, too much thermal energy,
etc. The excess thermal energy is removed from the combustion gases
in the exchanger, and they are cooled to the temperature required
for discharge to atmosphere.
[0186] The condensation of H.sub.2O simplifies the recycling and
recovery of the CO.sub.2, as the latter can be reused immediately
in the process. Its purity makes it a strategic product, by
substituting industrial CO.sub.2 which is generated by chemical
reactions on fossil materials and thus decreasing the impact of
greenhouse gases, etc.
[0187] A portion of the CO.sub.2 will be stored in a buffer tank,
under pressure, to maintain the capacity used in process start-up.
A portion can also be condensed by known systems, such as
freezing-out and/or compression: [0188] for cooling of wood at the
end of the roasting cycle [0189] for providing system safety:
liquid CO.sub.2 is a preferred means of neutralizing untimely
ignition of the wood.
[0190] The CO.sub.2 cycle can be described as follows: [0191] 1.
Production of CO.sub.2 by combustion of the carbon-containing
constituents of plant biomass under O.sub.2 as supporter of
combustion (C+O.sub.2.dbd.CO.sub.2); [0192] 2. Treatment and
purification of the combustion gas, CO.sub.2+H.sub.2O from the
combustion of H2 contained in the plant biomass fuel, [0193]
cooling and dehydration by a dedicated system such as a known
industrial dehumidifier, [0194] filtration of any unburnt particles
of biomass, using a filter that can be regenerated, and [0195]
preheating of the residual CO.sub.2, which thus becomes the gaseous
heat-transfer fluid for roasting, by looping on the
dehumidification system: thermal recovery of the latent heat of
condensation of H.sub.2O and of the energy for use in the
dehydration system; [0196] 3. Heating of the heat-transfer gas
stream, in the exchanger of the thermal generator; said stream,
preheated during the preceding treatment, has sufficient heat
capacity for appreciably lowering the temperature of the gas
mixture received from the post-combustion chamber, before it is
treated; [0197] 4. Dehydration of the wood to be roasted and
extraction of the gas mixture, composed of: [0198] the
heat-transfer stream (CO.sub.2) and [0199] H.sub.2O extracted from
the wet wood to be roasted; [0200] 5. Treatment of said gas mixture
through the "thermal base" of the generator; [0201] 6.
Post-combustion of components that are still combustible, in the
dedicated chamber, carried out by injection of O.sub.2; [0202] 7.
Transfer of energy to the heat-transfer gas stream after recycling
on passing through the heat exchanger [0203] 8. Treatment and
purification of this new gas mixture leaving the post-combustion
chamber; [0204] 9. Recycling of the residual CO.sub.2, which is
thus once again the heat-transfer fluid for roasting; [0205] 10.
Continuation of this cycle, until complete dehydration of the wood
to be roasted; [0206] 11. Cycle for roasting the dried wood and
extraction of the laden gas mixture, composed of: [0207] the
CO.sub.2 used as heat-transfer gas stream, and [0208] pyrolysis
gases extracted from the wood to be roasted (VOC); [0209] 12.
Treatment of the laden gas mixture in the thermal generator; and
[0210] 13. Post-combustion of constituents that are still
combustible and of the pyrolysis gases, in the dedicated chamber,
carried out by injection of O.sub.2; [0211] 14. Starting from point
11, the gases from the post-combustion chamber are recycled to the
roasting furnace up to the final stage of the operation without
passing through the system for treatment/dehydration/filtration.
Only a portion of these gases is treated and then compressed and
stored; and [0212] 15. Stopping the feed of heat-transfer CO.sub.2
and cooling of the roasted wood load by injection of liquid
CO.sub.2, which will draw its latent heat of evaporation from the
heat capacity of the load to be cooled: [0213] rotation of the
furnace 10 is maintained during this stage, as well as the
extraction of the evaporated and reheated CO.sub.2, [0214]
combustion of the "thermal base" is kept at idling, [0215] the gas
mixture, cooling CO.sub.2+combustion gas, is treated (see point 2),
and [0216] the residual CO.sub.2 is stored and/or liquefied.
[0217] Once the total CO.sub.2 capacity is reached, combustion
within the thermal generator can be carried out under atmosphere.
This situation does not prevail if the excess combustion gas is not
utilized in a total system with energy self-sufficiency or in
ancillary applications.
[0218] In an existing conventional method, with the starting wood
at 45% moisture (with lower heating value (LHV) of 7900 kJ/kg) it
is necessary to supply 3690 kJ per kilogram (kg) of raw material
for roasting it (latent heat of dehydration+sensible heat of
roasting). The 0.44 kg of roasted product originating will have an
LHV of 10 331 kJ (or an LHV/kg of 23 480 kJ/kg), which gives an
overall energy efficiency (except combustion efficiency linked to
the performance of the generator) of 10 331 kJ minus the 3690 kJ
consumed=6641 kJ per kg of wet raw material (B1).
[0219] In the method according to the invention, the water steam
extracted from the starting wood at 45% moisture is partly reduced
on passing through the "thermal base" of the generator. The
resultant gas mixture is thermally reactive: it holds, in its
formulation, the "exhaustive" principles of restitution of the
energy employed in the process. The combustion of these components
can thus be optimized in the post-combustion chamber, where heat
exchange with the heat-transfer gas stream is at its optimum:
[0220] losses in the system reduced to an acceptable level, and
[0221] markedly reduced temperature of the residual gas mixture
(combustion gas+H.sub.2O) before its transfer to the dehydration
system.
[0222] It can therefore be stated that with starting wood at 45%
moisture, we no longer consider the lower heating value of 7900
kJ/kg, as is the case in the conventional methods, but the HHV of
the components contained in the anhydrous wood, i.e. 23 600 kJ/kg
since: [0223] the method utilizes water steam produced as a
reactive thermal component, [0224] the method and the system
according to the invention are designed for recycling of CO.sub.2
and therefore recovery of at least a portion of the energy of
evaporation of the water contained in the raw material, [0225] the
method only consumes the energy required for compensating the heat
losses of the generator/roaster system.
[0226] Finally, a roasted product will be obtained, per kg of raw
material employed for roasting, LHV of which will also be 10 331
kJ, but as there is an excess in the energy balance of the process
(combustion of the VOC and direct utilization of the energy
generated) the process is considered not to have consumed anything
for the reaction.
[0227] One kilogram of raw material (starting wood) is therefore
upgraded from 7900 kJ to 10 331 kJ, or a gain of 30.77%.
[0228] Relative to the "conventional" methods of roasting (for
which the LHV of the amount of material roasted is disconnected
from the energy employed for the process, i.e. a final LHV of 6641
kJ), the gain is 55.58%.
[0229] Relative to forestry chips, supplied wet for feed to
installations for energy production using plant biomass, the
environmental benefits associated with the life cycle of the
roasted wood used as energy-producing wood, are: [0230] limitation
of atmospheric emissions just to the "neutral", excess CO.sub.2,
[0231] energy density is more consistent, allowing savings in
transport, storage and logistics of supplying thermal power
stations, [0232] absence of re-uptake of moisture during storage,
therefore smoothing out the constraints of using biomass "energy"
linked to seasonality and the risks of ignition of stocks, [0233]
improvement of the combustion efficiency of the thermal generator,
[0234] improvement of the overall thermal efficiency of the power
station, [0235] reduced investment for the systems employed for new
installations, and [0236] appreciable reduction in the amount of
raw material taken from the source, for an equivalent energy
return.
[0237] This system for roasting plant biomass can be arranged in a
battery of cylindrical assemblies, to satisfy the continually
varying demand for roasted wood.
[0238] The advantage of this system is that the cylindrical
roasting assemblies can be dimensioned for a standardized unit
capacity. Arranged in a battery, they will be supplied and
controlled by a single system for thermal generation/utilization of
the pyrolysis gases and one and the same system for management of
the CO.sub.2 produced.
[0239] The invention is not limited to the example that has just
been described, but can be applied to the roasting of all plant
biomasses.
* * * * *